1. Field of the Invention
This invention relates to an improved variable density aircraft, and more particularly to an autonomous aircraft capable of ready change from an aerodyne to an aerostat.
2. Description of the Prior Art
Aircraft are generally classified as aerodynes, that is heavier than air aircraft (HTA) or aerostats, that is lighter than air aircraft (LTA) according to their comparative density relative to the atmosphere.
The common characteristic of all aerodynes is that they must continuously expend energy to maintain steady state flight. Aerodynes consume relatively large quantities of energy because of their requirement for a continuous kinetic energy input to achieve and maintain flight.
The common characteristic of aerostats as represented by th rigid, non-rigid, semi-rigid airships and captive LTA gas balloons is that they maintain a density altitude by the potential energy of the LTA gases contained in their hulls. They float at their altitude of equilibrium. Actually, they are misnomered as they are neither HTA nor LTA, but in reality of the same density of the air in which they float. Powered aerostats consume relatively small quantities of energy as they do not require kinetic energy to overcome the force of gravity. Unfortunately, the density of the atmosphere is inconsistent and weight variations due to normal aerostat operations introduce density variations which must be compensated for as the density of an object is its mass to volume ratio. An aerostat's density may be varied by varying its mass or its volume.
Prior LTA gas aerostats use an unsatisfactory, irreversible mass variation system for aerostatic density change. Such systems involve either the concept of ballast dropping or the employment of gas valving to release gas to the atmosphere from the captured gas chambers or a combination of the two. This system of aerostatic vertical control was invented by Professor J. A. C. Charles in 1783 and incorporated into airships when they appeared on the scene. Its irreversibility has prevented the airship from attaining its full potential because it denies the airship autonomy. Airships using the mass variation vertical control system are hampered by their requirements of ballast aboard the airships, gas generation, storage and release systems and large ground crews at the departure points and destinations for literally holding down the airship. The airship must be driven from a higher altitude to a ground level by propulsive flight to the point where the ground crews grasp lines trailing from the airship to literally hold the airship adjacent the ground as a tethered aircraft.
The object of the present invention is to remove all of these constraints from an airship by the incorporation of an on board reversible variable density control system providing true autonomy to the aircraft and the creation of a variable density aircraft which may be readily changed from HTA to LTA and vice versa.
The mass variation system of Professor Charles has been in continued use for nearly two centuries because it has been generally accepted as a fact that the variation of the density of the aerostat volumetrically would incur prohibitive weight penalities due to temperature and/or pressure extremes involved in the significant volumetric reversible variations of most LTA gases. The most significant reversible volumetric variations in matter occur during their changes of state from liquid to vapor (vaporization/condensation) or from solid to gas (sublimation/solidification). Water, for example, varies from a liquid weighing 59.8 pounds per cubic foot to a gas weighing 0.0373 pounds per cubic foot during its vaporization at 212.degree. F (100.degree. C). This is a volumetric variation of 1,603 to 1. The problem with water as a reversible aerostatic vertical control system component is that it condenses too readily and requires a continuous input of heat energy to maintain it in the gaseous state. An airship employing such a system would in reality be an HTA aircraft.
However, an investigation of the lighter than air gases has shown ammonia as an LTA gas which has an exceptional affinity for water. One cubic foot of water at 20.degree. C under one atmospheric pressure (1ATM) will absorb approximately 700 cubic feet of ammonia. The same cubic foot of water at 32.degree. F (0.degree. C) under one ATM of pressure will absorb 1,250 cubic feet or 56.1 pounds of ammonia gas. Pure ammonia gas at 32.degree. F requires 4.23 ATMS to liquify and one cubic foot of this saturated liquid ammonia only weighs 39.87 pounds. Further, 1 cubic foot of saturated aqua-ammonia at 32.degree. F under 1 ATM contains 1.4 times as much ammonia as 1 cubic foot of the pure liquid at the same temperature but under more than four times as much pressure. This amazing solubility of ammonia in water is enhanced by the fact that light pressurization results in considerably larger concentrations of ammonia in the solution. The rate of absorption of the ammonia gas into the water varies with the amount of water surface area exposed to the gas, the temperature of the water and gas, and the pressure under which the solution is prepared. The rate of absorption may be very rapid where the gas is diffused into the water or the water is sprayed into the gas. An additional useful characteristic particularly to the present invention is that by elevating the heat of the solution to 100.degree. C, all of the ammonia gas is driven out of the solution and can be isolated separate from the water as a gas under normal atmospheric temperatues.
Investigations have shown that during the variation of temperature from 0.degree. to 100.degree. C, and the reduction of pressure from 2.37 ATMS to 1 ATM the volume of one cubic foot of saturated aqua-ammonia increases to 2,431 of gas and one of water. This represents a density variation of from 172 pounds per cubic foot to 0.0707 pounds per cubic foot which is 0.0047 pounds per cubic foot lighter than air. Thus, the present invention involves the discovery of the existence of a working fluid having its exceptional volumetric variability within relatively narrow temperature/pressure ranges closely related to the earth's environmental temperature/pressure ranges which permits the creation of a volumetric density variation system for aerostats which is both practical and realizable.
Applicant has developed a variable configuration, variable volume articulated vessel particularly applicable to the aircraft field which may advantageously change its configuration and volume to reduce aerodynamic drag and to meet the changing pressure of the air due to altitude variation. Such multiple section articulated vessel forms the subject matter of U.S. Pat. No. 3,661,290.
It is a further object of this invention to provide a volumetric density variation system for an aerostat formed of a variable configuration, variable volume articulated vessel constructed of relatively rigid panels hinge joined along rectilinear edges thereof.